#include <stdio.h>
int main()
{
    int arr[] = { 1,2,3,4,5,4,3,2,1 };
    int a = sizeof(arr) / sizeof(arr[0]);
    int r = 0;
    for (int i = 0; i < a; i++)
    {
         r = r ^ arr[i];
    }
    printf("%d", r);
    
    
  return 0;
}